This invention relates generally to an improvement in brazing methods and more particularly, but not by way of limitation, to an improved method of bracing metallic honeycomb sandwich structures.
Brazing of the structural elements of metallic sandwich structures, such as the cell nodes of honeycomb cores and cores to top and bottom or end panels may be carried out by a number of methods. Such methods include coating the cell walls and/or end panels with a brazing powder suspended in a binder, or placing thin sheets of brazing material between the core and the facing or end panels, or placing braze material at the core cell nodes or plating the core material and/or the facing or end panels with a suitable braze alloy. After one of the above methods of depositing braze materials is complete, the honeycomb core structure is sandwiched between the end panels in the form of a sandwich structure. The braze alloy may also be placed in the core by means of braze foil segments which are spot welded between the core nodes and which may bisect the cells of the core. Downwardly directed pressure and heat are then applied to the sandwich structure causing the braze material to melt and by capillary attraction flow to the cell nodes and the cell edge adjacent the end panels. When the sandwich structure is cooled an integral sandwich structure is formed.
Prior art braze bonding utilizing the above and other similar methods of braze bonding can be found in the following U.S. Pat. No. 3,030,703; No. 3,057,057; No. 3,068,565; No. 3,106,015; No. 3,656,224; and No. 4,333,598.
Such prior art braze bonding methods as have been heretofore described are capable of manufacturing generally acceptable sandwich structures. However, all such methods in some way or another are deficient in some respects and do not provide a braze bonding method that is truly efficient and economical and which provide a superior product.
For example, the method involving use of the braze powder requires application of an excess of braze material in order to ensure application of a requisite amount since uniform application is not possible. Application of additional braze material results in increased weight of the brazed panel and excessive use of an expensive braze material. In the heating phase of the braze process the acrylic binder bakes off and tends to contaminate the vacuum oven chamber and the associated diffusion pumps. Then if the oven and the pumps are not frequently cleaned the outgassing of the deposited binder on the contaminated interior of the oven and the pumps tends to oxidize and degrade the quality of the braze being made on further sandwich structure.
The powdered braze material is applied to the core by equipment that is subject to misalignment and miscalibration thereby resulting in unacceptable brazes. After the powdered braze material has been sprayed into the core the faying edges of the core must then be hand sanded to clean such edges of the braze powder and the sanded particles must also be removed in order to ensure an intimate fit between the faying edges of the core and the facing panels as required for capillary action and efficient distribution of the liquid braze material. It is evident that such operations are inefficient and time consuming.
The brazing method which involved the placing of braze sheets between the core and the face sheets is also deficient in that an excess of braze material is involved and the braze itself is also unacceptable. This is caused because the braze sheet melts before the pressure against the face sheets can increase to compensate for the gap left by the braze sheet going into a liquid state. Thus, a gap is encountered between the faying edges of the core and the face sheets and the braze material will not be drawn by capillary action into such gaps thereby resulting in an unacceptable weld.
The method using the braze foil segments spot welded into the nodes of the core is also inefficient since the braze material is part of the joint connecting the nodes of the core. Since this braze material is weaker than the material of the core, these joints at the core nodes tend to crack and break when the core is formed into a cylinder or other curved shape prior to being positioned between two mating face sheets to be brazed together. If these nodes separate the braze material will not bridge such gaps by capillary action in the brazing process and the resultant structure will be weak in those areas where the core joints had cracked. Also, when the braze material is provided by such braze foil segments secured to the core nodes, the foil segments tend to melt at one side of the foil segment bisecting the cell and the remaining portion of the foil tends to melt into the joint away from the end that initially melted and the resulting braze panel is not uniform in strength. If the foil segments are pre-clipped in a labor intensive step, the foil segment held within the node melts before the remaining portion of the foil segment with the remaining portion falling downwardly to the cell floor and resulting in an excess of the braze material on the melted area of one face sheet as opposed to the other face sheet. It is believed that such difficulties as have been described are obviated by the present novel method.